Nowadays near-infrared (NIR) absorbing dyes find use in many applications, typically in electronic materials and the demand for these dyes are increasing. They are useful in many industrial fields. Specifically they cover widely varying applications including filter dyes in optical filters and heat-absorbing filters, additives to image-forming materials using infrared-sensitive coating and solder resists, sensitizers for photo-polymerization and photo-crosslinking reaction, and optical recording materials for optical discs.
While plasma display panels (PDP) are attractive as the large-size flat display, electromagnetic waves and NIR light inevitably generate in principle. The PDPs need means for shutting out interference, for example, electromagnetic wave shields and NIR absorbing filters. These means ameliorate imaging failures and prevent malfunction of remote controllers by infrared interference. JP-A 2000-81511 discloses NIR absorbing dyes which are intended for use in optical filters.
Another example is a CCD camera wherein a NIR absorbing filter is used as an optical filter for shielding off incident NIR light. Then the camera is given a spectral sensitivity approximate to the visible sensitivity. The optical filters to this end often use NIR absorbing dyes such as cyanine dyes, phthalocyanine dyes, and diimonium salts. With respect to the optical filter containing NIR absorbing dye for use in CCD cameras, reference is made to JP-A H11-23837, for example.
Also NIR absorbing dyes are used in invisible printing ink. Printing with ink having no absorption band in the visible light region makes information codes indiscernible by visual observation, thus preventing forgery of confidential documents such as securities and bonds. Reference may be made to JP-A H10-60409.
NIR absorbing dyes are applied to the fabrication of semiconductor devices as well. In the micropatterning step during optical lithography, a photoresist film is exposed to radiation from the projection optical system while optical auto-focusing sensor NIR light is performed so that the wafer surface may be in register with the best image plane of the projection optical system, that is, so as to enhance focus. To increase the accuracy of optical auto-focusing, U.S. Pat. No. 5,643,700 proposes a photoresist film containing a NIR absorbing dye. US 20090208865 discloses a method for introducing a NIR absorbing dye-containing film beneath a photoresist film.
While NIR absorbing dyes are useful in numerous applications as discussed above, one common problem is solubility in organic solvents. While typical NIR absorbing dyes include cyanine dyes, phthalocyanine dyes, metal complexes of nickel or the like, and diimonium salts, they tend to have poor solubility in many organic solvents. Most NIR absorbing dyes are used in organic systems containing base resins and additives as well as the dyes. Reduced solubility of NIR absorbing dyes in organic solvents limits the field of utility thereof. In the IR filter for PDP, for example, a cyanine compound having hexafluoroantimonate as an anion is used which has low solvent solubility despite the advantage of heat resistance. In addition, compounds free of a heavy metal such as antimony are needed in the field of electronic materials where a reduction of metal impurities is desired. WO 2006/006573 describes heavy metal-free dye compounds, which are less soluble in coating solvents such as methyl ethyl ketone. U.S. Pat. No. 5,541,235 describes dyes having a fluorinated alkylsulfonyl anion. However, use of perfluoroalkyl compounds is undesirable because their stability (anti-degradation) assigned to C—F bonds and the biological concentration and accumulation due to hydrophobic and lipophilic properties are of concern. Furthermore, JP-A 2008-88426 describes that solvent solubility is improved using tris(trifluoromethanealkylsulfonyl)methide anion. This anion material is expensive and not regarded commercially viable.
In most cases, it is possible to dissolve NIR absorbing dyes if highly polar solvents such as methanol and dimethyl sulfoxide are used. The use of highly polar solvents, however, may produce undesired side effects such as formation of an uneven film and crystallization as a salt in a dry film. In addition, high-boiling solvents such as dimethyl sulfoxide are difficult to evaporate off during film formation.
Although it is possible to form a film, without a need for organic solvents, by kneading a NIR absorbing dye with a matrix material such as a resin or the like and shaping the mix into a film, this approach is awkward to form a uniform film. On the other hand, if a NIR absorbing dye-containing material dissolves in an organic solvent, it is easy to form a uniform film by standard film-forming techniques such as spin coating. This material may be shelf stored in solution form and easy to handle. For these reasons, NIR absorbing dyes having good solvent solubility offer a great advantage to the film forming process.